1. Field of the Invention
The present invention relates to a means for transmitting data on the phase of speech and, more particularly, a means for embedding data into speech signals, with only a minimal increase in the signal bandwidth, by performing a discrete Fourier transform (DFT) on blocks of speech samples and replacing the resulting phase components of selected portions of the speech transform with the data which is desired to be transmitted.
2. Description of the Prior Art
There exist numerous systems which attempt to multiplex digital data with an analog signal. One early technique for voice-data multiplexing, diclosed in U.S. Pat. No. 3,304,372 issued to R. F. J. Filipowsky et al on Feb. 14, 1967, relates to a voice-data multiplexing system for transmitting data during the pauses in the voice signal. As taught in Filipowsky et al, voice signals are normally gated onto a transmission channel at the transmitter, but whenever gaps exceeding a minimum interval are sensed in the voice signals, a data source is activated and data transmission begins with the dispatch of a keying signal over the transmission channel.
There also exist methods of incorporating digital data with microwave analog signals, variously termed data-above-voice (DAV), data-under-voice (DUV) and data-above-video (DAVID). The former two systems are described in the article "1.544 Mbit/s Data Above FDM Voice and Data Under FDM Voice Microwave Transmission" by K. Feher et al in IEEE Transactions on Communications, November 1975, at pp. 1321-1327, while the latter method is described in the article "Simultaneous Transmission of Digital Phase-Shift Keying and of Analog Television Signals" by K. Feher et al in IEEE Transactions on Communications, December 1975, at pp. 1509-1514. As described in both articles, the data is transmitted in the unused portion of the spectrum, either below or above that portion dedicated to the microwave voice or video signal.
Methods also exist for utilizing Fourier transform techniques in association with speech signals prior to transmission. U.S. Pat. No. 3,681,530 issued to H. J. Manley et al on Aug. 1, 1972 relates to a bandwidth compression system wherein the discrete Fourier transform of the logged spectrum magnitudes of a speech signal are obtained to form the Fourier transform of the logarithm of the frequency spectrum magnitude (FTLSM) of the input speech. An encoding unit selects and encodes certain ones of the FTLSM coefficients for transmission to a remote terminal for analysis. The encoded signals include pitch data and vocal tract impulse data, both of which are derived from the FTLSM signals.
The discrete Fourier transform is also used in many vocoder arrangements, one of which is described in the article "Channel Vocoder Based on C. C. D. Discrete-Fourier-Transform Processors" by M. C. Davie appearing in IEE Proceedings, Vol. 127, Part F, No. 2, April 1980 at pp. 132-143. The Davie paper discusses the implementation of a central speech processor based on a chargle-coupled-device (c.c.d.) discrete Fourier transform algorithm which is employed to provide spectral data for channel compression and cepstral data for pitch detection.
It is also known in the art to employ Fourier transform techniques to achieve speech compression. One article describing a particular technique is "Speech Compression by Means of the Discrete Fourier Transform" by J. Shulman, appearing in Proceedings of the Ninth Convention of Electrical and Electronics Engineers in Israel, Apr. 22-24, 1978 at p. A-2-4. The method as described involves sampling of the speech signal, calculation of the discrete Fourier transform, selection of dominant spectral lines (according to a local maxima algorithm) and transmission of the selected lines. At the receiving end, reconstruction is performed by inverse transforming (IDFT) the received components, with zeroes inserted in place of the missing components.
There exist a limitation in all of the above-cited multiplexing references wherein the bandwidth of the speech signal is inevitably increased by the addition of data thereto, thus rendering communication over limited-bandwidth channels very difficult. A problem remaining in the prior art, therefore, is to provide a means for multiplexing data and speech without incurring an excessive expansion of the speech bandwidth.